The present invention relates to a zoom lens system, and more particularly to a compact zoom lens system suitable for use in a 35 mm lens shutter camera. The zoom lens system comprises only five lens elements.
Generally, the zoom lens systems fall into two categories. The first category includes the retrofocus type zoom lens system, which typically comprises a front lens group having a negative refractive power and a rear lens group having a positive refractive power. An airspace is reserved between the front lens group and the rear lens group, and it is possible to effectuate variation of focal length of the lens system by varying the airspace. Such a zoom lens system has characteristics which enable reservation of enough space for arranging a mirror. Consequently, the first type of zoom lens system is suitable for use in the well-known single lens reflex camera. However, it has disadvantages in that the front lens group of such a lens system has a large diameter and long total length, and these disadvantages make it impossible to make the zoom lens system compact.
The second category of the zoom lens system is the telephoto system, which typically comprises a front lens group having a positive refractive power and a rear lens group having a negative refractive power. An airspace is also reserved between the front lens group and the rear lens group for zooming. This type of zoom lens system has a shorter back focal length and therefore allows the total length of the zoom lens system to be shortened, it is especially suitable for use in lens shutter cameras.
To overcome the problems above, various prior art compact zoom lens systems have been developed in this field. For example, U.S. Pat. No. 4,772,106 discloses a compact zoom lens system which is comprised of a first lens group having a positive refractive power and a second lens group having a negative refractive power in the order from the object side. The first lens group of such a lens system comprises at least two positive lenses and at least one negative lens, and the second lens group comprises a positive lens and a negative lens.
U.S. Pat. No. 4,818,081 discloses a lens system which employs a two group type configuration composed of a first lens group having a positive focal length and a second lens group having a negative focal length.
U.S. Pat. No. 4,830,476 discloses a compact zoom lens system comprising four lens elements in the lens unit. The front lens group of the zoom lens system comprises a negative lens element and a positive lens element, and the rear lens group comprises a positive lens element and a negative lens element.
U.S. Pat. No. 4,838,669 discloses a zoom lens system which has a positive refractive power and a rear lens group with a front lens group which has a negative refractive power. The front lens group of the lens system comprises a positive meniscus lens, a negative lens and at least one positive lens, and the rear lens group consists of a positive lens, a biconcave lens and a negative meniscus lens.
U.S. Pat. No. 4,682,860 discloses a zoom lens system which is comprised of a front lens groups having a positive refractive power and a rear lens group having a negative refractive power. The first lens group includes at least two positive lens elements and at least one negative lens element; the second lens group includes at least one positive lens element and at least one negative lens element. It is noted that the fourth embodiment as shown in FIG. 9A and 9B of the '860 patent comprises five lens elements forming a zoom lens system. However, the lens system disclosed in the '860 patent contains lens elements with relatively steep radii of curvature and thus is relatively costly to manufacture.
U.S. Pat. No. 4,991,945 discloses a zoom lens system comprising five lens elements. The first three elements form a positive lens group and the last two elements form a negative lens group. All of the lens elements in this prior patent are spherical and the optical condition/parameters such as radii of curvature of lens elements are different from the present invention.